This invention is directed to brazing filler metals of nickel-palladium-based alloys which contain the transition metal iron to produce an alloy which is particularly useful for brazing a wide variety of materials at high temperatures, and results in the production of high strength brazements. More particularly, the invention is directed to quaternary alloys of nickel-palladium-iron-silicon having liquidus temperatures of at least about 980.degree. C.
Brazing is a process of joining metal parts, often of dissimilar composition, to each other. Typically, a brazing filler metal that has a melting point lower than that of the parts to be joined is interposed between the parts to form an assembly. The assembly is then heated to a temperature sufficient to melt the brazing filler metal. Upon cooling, a strong, preferably corrosion resistant, joint is formed.
Brazing filler metals consisting of nickel-palladium-based alloys have been developed which exhibit high temperature strength, good corrosion resistance and good erosion resistance. Such alloys have been disclosed, for example, in U.S. Pat. Nos. 4,508,257, 4,405,391 and 4,448,618. The alloys disclosed in these patents, however, each exhibit drawbacks which make them unsuitable for brazing products which require prolonged service life at high temperatures. The alloys disclosed in U.S. Pat. Nos. 4,405,391 and 4,448,618 contain boron in substantial amounts. It is well known that boron diffuses extensively out of the joint area into to stainless steel and superalloy base metals when subjected to brazing at very high temperatures and, also, during prolonged service of the brazed joint at elevated temperatures. Specifically, boron, with its small atomic radius, diffuses along grain boundaries forming therein intermetallic borides and resulting in brittle fracture under loading at elevated temperatures. Therefore, alloys containing boron, in spite of having rather high melting characteristics, are not suitable for brazing products designed to withstand high temperature environments, for example, stainless steel and/or superalloy honeycomb structures employed in aircraft wings. Regarding the ternary nickel-palladium-silicon alloys disclosed in U.S. Pat. No. 4,508,257, these alloys have large concentrations of nickel and silicon but contain essentially no boron. Although the problem associated with boron is avoided, the high nickel and silicon content results in a rather low melting temperature product. Specifically, these alloys have liquidus temperatures in the range of about 877-948.degree. C. Therefore, these alloys are not effective for use in brazing products to be employed in high temperature applications.
Most recently, a new class of nickel-palladium-based alloys were developed (see U.S. ser. No. 89,276, filed Aug. 25, 1987 and commonly assigned now U.S. Pat. No. 4,746,375), which are substantially free of boron. However, these alloys were developed to overcome problems associated with brazing cemented carbide parts and, unfortunately, are unsuitable for high temperature brazing and for use in structures employed in high temperature environments due to their low melting point (less than about 950.degree. C.).
Accordingly, there remains a need in the art for brazing filler materials suitable for brazing stainless steels and superalloys at high temperatures and which will result in brazements which retain high strength at elevated temperatures over prolonged periods of time.